WO2004025793A1

WO2004025793A1 - Laser driver control circuit

Info

Publication number: WO2004025793A1
Application number: PCT/EP2003/008324
Authority: WO
Inventors: Raymond A. Filippi; Hans Joakim BÄNGS
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2002-09-12
Filing date: 2003-07-28
Publication date: 2004-03-25
Also published as: GB2393034B; GB0221194D0; GB2393034A; AU2003251654A1

Abstract

A dual loop laser control circuit using a single preset resistor and a photo diode to control a laser diode (together with the photo diode forming a laser submodule). The coupling coefficient of the laser submodule is used to determine the value of the pre-set resistor prior to circuit assembly. Subsequent trimming of the pre-set resistor is not required as the circuit automatically adjusts for changes in laser efficiency and laser threshold. A data pattern from the input data is utilised in the feedback loops and compared to a data pattern received by the photodiode, thus avoiding data pattern sensitivities. This circuit can be integrated onto a single integrated circuit.

Description

LASER DRIVER CONTROL CIRCUIT

Technical Field of the Invention

The present invention relates to an opto-electronic circuit, and in particular to a laser driver control circuit that can be integrated onto a single integrated circuit.

Description of Related Art

Generally, a laser driver control circuit used in high data rate transmission should simultaneously maintain constant average optical output power and constant peak optical output power, independent of the effects of temperature, ageing and data patterns. It is_^ necessary to maintain these power levels, because where the laser is biased below the threshold of stimulated emission, a turn on delay occurs and a relaxation oscillation can occur during turn on, resulting in a high bit error rate. The graph in Figure 1 illustrates a typical laser characteristic with a slope of spontaneous emission coefficient 10 below a threshold point of stimulated emission 12 and a slope of stimulated quantum emission coefficient 14 (known as laser efficiency) above the threshold point of stimulated emission 12.

Figure 2 illustrates the degradation in both the threshold of stimulated emission and laser efficiency that occur with increasing operating temperatures. Specifically, the graph illustrates laser characteristics at operating temperatures of -40°C, 25°C and 85°C, indicated by reference numerals 20, 22 and 24 respectively. In fibre optic networks, light is launched into a fibre from a front facet of a laser diode. This output signal comprises a pattern of level one optical output power (representing binary l' of the input signal to the laser diode) and a level zero optical output power (representing binary ^λ0'). In order to control the optical output power of the laser diode a feedback system is typically utilised. This is achieved by positioning a photodiode in close proximity to the rear facet of the laser diode, so that the photodiode detects a proportion of the emitted light. .The photodiode output is fed back into the laser driver control circuit and is utilised in the control of the magnitude of the current driving the laser diode. The combined components of the laser diode and the associated monitoring photodiode are often referred to as a sub-module. A parameter called the coupling coefficient relates the photodiode output current to the laser output power in such a sub-module. The coupling coefficient is dependent upon several factors, including the proportion of light emitted at the rear facet of the laser diode, the responsivity of the photodiode, the alignment of the laser diode and the photodiode and the alignment of the optical fibre to the front facet of the laser diode.

To determine the coupling coefficient of a sub-module it is necessary for the relevant parameters to be measured during the manufacturing process. The coupling coefficient does not vary significantly with age or operating temperature. The latter is due to the minimal temperature coefficient of a photodiode. The coupling coefficient is measured in 7Λmps per Watt and for a typical long haul laser system with average optical output power of 0.5mW to 2mW, a usual coupling coefficient is 0.05 A/W to 1 A/W respectively. A further parameter associated with an optical transmitter is the extinction ratio, i.e. the ratio of light intensity emitted from the laser diode at logic one level to the light intensity emitted at logic zero level (or equally, the ratio of the one level current to the zero level current in the photodiode, as this is linear) .

A conventional technique employs single loop control to maintain the average optical output power at a constant level. For example, see ^λ0ptical Fibre Communication Systems' edited by C. P. Sandbank, John Wiley & Sons, 1980, page 210, figure 157. The disadvantages associated with this technique include the requirement for trimming the value of the modulation current and average power during transmitter module construction. Further, several years into the life of the device, the need could arise to retrim the powerset resistor to compensate for changes in the laser diode due to ageing. Where the laser driver control circuit is implemented in an uncooled laser application, the degradation of the laser diode due to temperature must be considered. Conventional prior art techniques include the implementation of a further external preset resistor in addition to that already indicated. It is also known in such prior art devices to include a further trim to achieve data pattern insensitivity.

Where a laser driver control circuit functions simply to maintain constant average optical output power, difficulties can arise. Average optical output power is maintained if the logic one level light falls and the zero level light increases, but this leads to uncontrolled modulation in the logic one and zero level light. These known laser driver control circuits are also sensitive to data patterns, i.e. when long strings of ones or zeros occur in the data input signal. These cause modulation of the peak level optical output power optical or zero level optical output power and result in non-constant average optical power and sensitivity to subsequent data input signals.

Another conventional technique employs a dual loop control circuit to maintain the average optical output power at a constant level. Dual loop control circuits require a high speed photo diode and a high bandwidth feedback path. The disadvantages associated with this technique include insufficient dynamic range, inability to process a wide magnitude range of optical feedback signal and the requirement for manual trimming during device construction.

It is known from "A digitally programmable burst-mode 155Mb/s transmitter for PON", Doci, et al, pp212-215, ESSCIRC Conference 2000, to mitigate the problem of insufficient dynamic range by the implementation of variable gain transimpedance amplifiers (TIAs) . For example, where programmable TIAs are used within a dual loop control circuit, a second micro-controller integrated circuit (pre-programmed with the laser diode characteristics) needs to be included in the application circuit, although such a circuit mitigates the requirement for manual trimming during device construction.

Therefore, the present invention seeks to provide a laser driver control circuit in which problems such as insufficient dynamic range, data pattern insensitivity, and the need for manual trimming, are at least alleviated.

Summary of the Invention

According to a first aspect of the present invention, there is provided a control circuit for a laser diode producing a laser output signal. The control circuit comprising an input for receiving a current from a photodiode optically coupled to a laser diode, and an output for supplying a driving signal to the laser diode, and further comprising means for deriving first and second measured values from the received current, corresponding respectively to data ones and zeros in the laser output signal, means for setting first and second reference values, corresponding respectively to a data one and a data zero, and feedback circuitry for controlling the driving signal such that the first and second measured values become equal to the first and second reference values respectively, wherein the means for setting first and second reference values comprises means for providing a reference current, means for selecting a desired ratio between said first and second reference values, and means for setting first and second reference values based on said reference current and said desired ratio between said first and second reference values.

According to a second aspect of the present invention, there is provided a signal processing element including the control circuit of the first aspect of the present invention. According to a third aspect of the present invention, there is provided a transceiver for an optical fibre network including the control circuit of the first aspect of the present invention.

According to a fourth aspect of the present invention, there is provided an optical drive circuit, comprising the control circuit of the first aspect of the present invention and a laser sub-module, comprising a laser diode and a photodiode optically coupled thereto.

Advantageously, the present invention requires only a single pre-set resistor and can be integrated onto a single integrated circuit, which leads to relatively low production costs. As the laser efficiency and the threshold point of spontaneous emission change with temperature and ageing, automatic adjustments are made to simultaneously maintain a constant extinction ratio and average optical output power from the laser. Hence, the invention can be utilised in both uncooled and cooled laser applications. Further advantage is gained in that the invention has sufficient dynamic range to function with a wide variety of laser optical output powers and coupling coefficients.

Also, data pattern insensitivity is avoided by comparison of input data in the feedback control loops and input data received from the photodiode. Further, the invention continues to be immune to data pattern sensitivities at very low data rates.

Brief Description of the Drawings

For a better understanding of the present invention, and to show how it may be put into effect, reference will now be made, by way of example, to the accompanying drawings in which:

Figure 1 shows a graph of a typical laser characteristic plotting laser current on the abscissa versus average Optical Output Power (OOP) on the ordinate;

Figure 2 shows a graph of a typical laser characteristic for several temperatures; and

Figure 3 is a schematic diagram of a laser driver control circuit in accordance with an embodiment of the present invention.

Figure 4 is a schematic diagram of a laser driver control circuit in accordance with a second embodiment of the present invention. Figure 5 is a schematic diagram of a laser driver control circuit in accordance with a third embodiment of the present invention.

Detailed Description of Preferred Embodiments

In the laser driver control circuit 30 of Figure 3, extinction ratio inputs 32, 33, 34 are coupled to a reference generator 36. A power preset resistor 38, of value R, is coupled to a reference control 40 and a bandgap reference circuit 41.

The required value, R, for the power pre-set resistor 38 is determined prior to placement in the laser driver control circuit 30 during the manufacturing process. Three parameters are initially identified; namely, desired average optical output power (P_AV) _r desired extinction ratio (Ex) and the coupling coefficient (77) of the laser driver control circuit. It should be noted that, where Pi denotes the logical level one optical output power, it is related to the extinction ratio by the following equation:

Where I_AV denotes the average current output fed back from the laser sub-module to the control circuitry, then :

Since the reference control block 40 is intended to generate a reference current which is equal to I_AV, the required resistance value is calculated, where V_REF denotes the reference voltage, utilising Ohm's Law to give:

V RBF

= R 1 AV

A reference average current from reference control 40 is input into the reference generator 36.

The dc reference currents, namely a zero level reference current 42, an average level reference current 44, and a one level reference current 46 are output from the reference generator circuit block 36 and fed into a zero level transimpedance amplifier (TIA) 48, an average level TIA 50, and a one level TIA 52, respectively. Specifically, the average level reference current 44 is determined by the value of power preset resistor 38 and reference control 40. The extinction ratio inputs 32, 33, 34 include two inputs 32, 34 which receive a two-bit digital signal, and the value of this two-bit signal selects one of four preprogrammed absolute values (for example, 11, 16, 21 and 26) which are calculated to enable a certain one level reference current I_ to be output on line 46 by the reference generator circuit block 36, utilising the following equation:

The extinction ratio can be further controlled by an analogue input in addition to the digital inputs. Such additional analogue control can be in the form of an "adjust" input and can be implemented as an input 33 to the reference generator circuit block 36. In practice, the analogue input 33 can be, for example, a voltage sensing input, a current sensing input, or an impedance sensing input. The reason for including the additional analogue control is to increase the level of resolution. For example, in a circuit where the four pre-programmed extinction ratios are 11, 16, 21 and 26, the analogue input can preferably adjust this ratio by ±5x, allowing any extinction ratio from 6 to 31 to be attained.

Alternatively, the extinction ratio can be controlled either by an analogue input, or by digital inputs, alone .

The zero level reference current I. on line 42 is calculated using the following equation;

Again, the reference generator circuit block 36 implements this equation. The current generated by the reference control block 40 is also input to an analogue-to-digital converter 54, which generates a digital output. This digital output programmes the required gain in the TIAs 48, 50, 52 and also in a fourth TIA 56, referred to as the live TIA. The live TIA 56 receives an input from a photo-diode 58. This is done such that the TIAs 48, 50, 52 and the fourth TIA 56 have the same gain.

Moreover, programming the gain of the TIAs on the basis of the current generated by the reference control block 40 ensures that the feedback loops have a wide dynamic range.

The zero level TIA 48, the average level TIA 50 and the one level TIA 52 output a zero level voltage 60, an average level voltage 62 and a one level voltage 64, respectively.

The photodiode 58 detects an optical data bit stream generated from a rear facet of a laser diode 66, the laser diode 66 and photodiode 58 together forming a laser sub-module 67. Hence, the live TIA 56 outputs an ac voltage 68, the live voltage, which is representative of the optical data bit stream. This output from the live TIA 56 is single ended and is converted to a differential signal using the average level voltage 62 as a signal ground.

A reference modulator 70 receives the zero level voltage 60, the average level voltage 62, the one level voltage 64, the live voltage 68 and also a positive data input 72 and a negative data input 74. The positive data input 72 and negative data input 74 receive the input data bit stream 114 which is to be used to modulate the laser output. The reference modulator output consists of four pairs of differential signals 76, 78, 80 and 82 which are fed through low pass filters 84, 86, 88, 90, respectively, in order to equalise the frequency response prior to signal processing.

The first and third pairs of differential signals 76, 80 are derived from the optical feedback path via the photodiode 58. More particularly, the first pair of differential signals 76 represents a measured value obtained from the photodiode current when the input data value is a data one, while the third pair of differential signals 80 represents a measured value obtained from the photodiode current when the input data value is a data zero.

The second and fourth pairs of differential signals 78, 82 are the reconstructed reference bit stream. More particularly, the second pair of differential signals 78 represents a reference value obtained from the one level reference current, while the fourth pair of differential signals 82 represents a reference value obtained from the zero level reference current.

A first high speed peak detector 92 compares the first pair of differential signals 76 after filtering and similarly, a second high speed peak detector 94 compares the second pair of differential signals 78 after filtering. An output from both the first high speed peak detector 92 and the second high speed peak detector 94 are then compared in a first differential charge pump 96.

The first and second pairs of differential signals 76, 78 form a first part of the feedback loop which is associated with the signal peak value, known as a Peak Level Locking Loop (PLLL) , and functions to control a modulation current. The third and fourth pairs of differential signals 80, 82 form a second part of the feedback loop which is associated with the signal bottom value, known as a Bottom Level Locking Loop (BLLL), and functions to control a laser bias current.

A first high speed bottom detector 98 compares the third pair of differential signals 80 after filtering and similarly, a second high speed bottom detector 100 compares the fourth pair of differential signals 82 after filtering. An output from both the first high speed bottom detector 98 and the second bottom detector 100 are then compared in a second differential charge pump 102.

An output voltage of the first differential charge pump 96 is a modulation level control voltage 104 and provides a measure of peak value. This voltage 104 is stable when the PLLL is locked.

An output voltage of the second differential charge pump 102 is a bias level control voltage 106 and provides a measure of bottom value. This voltage 106 is stable when the BLLL is locked.

The modulation level control voltage 104 drives a linear to logarithmic voltage to current converter 110, passing via a PLLL filter capacitor 109. The resultant modulation level control current is multiplied by a fixed constant by multiplying means 112. The input data bit stream 114 determines when the modulation level control current is switched to modulate the laser diode 66 by sinking current from the laser cathode. Similarly, in the BLLL, the bias level control voltage 106 drives a linear to logarithmic voltage to current converter 118, passing via a BLLL filter capacitor 117. The resultant bias level control current is multiplied by a fixed constant by multiplying means 120 and then coupled to the laser diode 66.

The voltage to current converters 110, 118 of the present embodiment have an exponential characteristic. This means that a change in the input voltage will result in a change in the output current that is predetermined percentage change from a previous output current. Use of this type of linear to logarithmic voltage to current converter 110, 118, enables the laser driver control circuit 30 to have a similar response to noise at all operational levels and increased stability (in particular when using lasers of differing quantum efficiencies) .

In operation, light emitted from a rear facet of the laser diode 66, detected by the photo diode 58, is converted into a monitoring current 122 which is the feedback signal. This high frequency, ac monitoring current 122 is input to the low input impedance live

TIA 56, and hence, determines the a.c. voltage 68. The three reference voltages, zero level voltage 60, average level voltage 62 and one level voltage 64 are determined by the value of the power preset resistor 38, the extinction ratio digital programmable inputs 32, 34, and the reference generator circuit block 36. The a.c. data zero level from the live TIA 56 is forced to match the zero level voltage 60 and the a.c. data one level from the live TIA 56 is forced to match the one level voltage 64 by the negative feedback loop increasing or decreasing light output of the laser diode 66.

Importantly, when the laser diode 66 has a constant laser characteristic, the modulation level control voltage 104 and the bias level control voltage 106 on the PLLL linear to logarithmic voltage to current converter 110 and the BLLL linear to logarithmic voltage to current converter 118, respectively, remain constant and both the PLLL and BLLL become locked to the one and zero levels respectively.

When the laser characteristic of laser diode 66 changes with temperature change or ageing, the PLLL and BLLL will compensate for this change by locking to the new balance condition. This is achieved by setting a new modulation level control voltage 104 and a new bias level control voltage 106 on the PLLL linear to logarithmic voltage to current converter 110 and BLLL linear to logarithmic voltage to current converter 118, respectively. This also maintains a constant output optical power and extinction ratio.

Figure 4 is a schematic diagram of a laser driver control circuit in which common reference numerals have been employed where common circuit elements have the same function as in the circuit of Figure 3. Modification is found in the BLLL, where low pass ^• filters 88, 90 and high speed bottom detectors 98, 100 can be replaced by two low pass filters 124, 126. Each low pass filter is coupled to reference modulator 70 by a single connection 128, 130, respectively. Alternatively, the PLLL can be modified in a similar way.

In operation, the modified circuit of Figure 4 functions in a similar way to the circuit depicted in Figure 3. The output of low pass filters 124, 126 drives differential charge pump 102 to determine the bias control voltage for the BLLL. Therefore, this modified architecture compares the integrated average bottom level signal rather than the actual bottom level signals. Where it is the PLLL that is modified, then the integrated average peak level signal is compared rather than the actual peak level signals.

Figure 5 illustrates a laser driver control circuit in accordance with a third embodiment of the present invention. Again, common reference numerals have been employed where common circuit elements have the same function as in the circuit of Figure 3. Modification ' is found in the use of three TIAs, namely a live TIA

56, a reference TIA 136 and a ground TIA 50, as opposed to four TIAs in the embodiment of Figure 3. Specifically, zero level reference current 42 and one level reference current 46 are fed into a second reference modulator 132, which also has a positive data input 172 and negative data input 174 which receive the data signal used to modulate the laser output. The output 134 of the second reference modulator feeds the reference TIA 136 and the current is switched according to the data input signal pattern. Thus, when the data signal is a data zero, the second reference modulator outputs the zero level reference current 42 and, when the data signal is a data, the second reference modulator outputs the one level reference current 46.

The first reference modulator 70 can then provide the appropriate outputs to the two feedback loops as in the embodiment of Figure 3, such that the measured values, obtained when the data input has values zero and one respectively, are compared with the reference values obtained from the zero level reference current and the one level reference current.

It will be apparent to the skilled person that the above described circuit architecture is not exhaustive and variations on this structure may be employed to achieve a similar result whilst employing the same inventive concept. For example, the circuit of Figure 5 can be modified to function with only two TIAs. Specifically, where a suitable signal ground reference signal is provided, the ground TIA can be omitted. Also, the circuit architecture could be modified for using a laser diode 66 with a grounded cathode connection by changing the polarity of the output current drive from sinking to sourcing.

It can therefore be seen that the present invention provides a laser driver control circuit which has significant advantages over conventional devices.

Claims

1. A control circuit for a laser diode producing a laser output signal, comprising: an input (at 56) for receiving a current from a photodiode optically coupled to a laser diode; and an output (112, 120) for supplying a driving signal to the laser diode; and further comprising: means (70) for deriving first and second measured values from the received current (76, 80) , corresponding respectively to data ones and' zeros in the laser output signal; means (70) for setting first and second reference values (78, 82), corresponding respectively to a data one and a data zero; and feedback circuitry for controlling the driving signal such that the first and second measured values become equal to the first and second reference values respectively, wherein the means for setting first and second reference values comprises: means (40) for providing a reference current; means (32, 34) for selecting a desired ratio between said first and second reference values; and means (60,62) for setting first and second reference values based on said reference current and said desired ratio between said first and second reference values.

2. A control circuit as claimed in claim 1, wherein the feedback circuitry further comprises means for receiving a data input signal comprising data ones and zeroes .

3. A control circuit as claimed in claim 2, wherein the feedback circuitry further comprises means for comparing the first measured value to the first reference value, and for controlling the driving signal such that the first measured value becomes equal to the first reference value, when the data input signal is a data one.

4. A control circuit as claimed in claim 2, wherein the feedback circuitry further comprises means for comparing the second measured value to the second reference value, and for controlling the driving signal such that the second measured value becomes equal to the second reference value, when the data input signal is a data zero.

5. A control circuit as claimed in claim 1, wherein the means for selecting a desired ratio between said first and second reference values comprises means for selecting between several pre-selected available ratios .

6. A control circuit as claimed in claim 1, wherein the means for selecting a desired ratio between said first and second reference values comprises means for receiving an input representing a value for said desired ratio.

7. A control circuit as claimed in claim 6, wherein the input representing a value for said desired ratio comprises a digital input.

8. A control circuit as claimed in claim 6 or 7, wherein the input representing a value for said desired ratio comprises an analogue input.

9. A control circuit as claimed in claim 1, wherein the feedback circuitry comprises a first feedback loop for controlling the driving signal such that the first measured value becomes equal to the first reference value, and a second feedback loop for controlling the driving signal such that the second measured value becomes equal to the second reference value.

10. A control circuit as claimed in claim 9, wherein the first and second feedback loops each comprise a pair of filters and a differential charge pump.

11. A control circuit as claimed in claim 10, wherein the first feedback loop further comprises a peak detector and a voltage to current converter with an exponential characteristic.

12. A control circuit as claimed in claim 10, wherein the second feedback loop further comprises a bottom detector and a voltage to current converter with an exponential characteristic.

13. A control circuit as claimed in claim 1, wherein the means for setting first and second reference values further comprises a resistor for providing the reference current from a reference voltage.

14. A control circuit as claimed in claim 1, comprising: a first transimpedance amplifier, for converting the current received from the photodiode into a voltage signal; at least one second transimpedance amplifier, for deriving the first and second reference values from said reference current and said desired ratio between said first and second reference values; and means for controlling the gains of said first and the or each second transimpedance amplifier, based on a level of said <reference current.

15. A control circuit as claimed in claim 1, wherein the means for setting first and second reference values comprises : means for receiving a data input signal comprising data ones and zeroes; means for providing first and second reference current values from said reference current and said desired ratio between said first and second reference values; a second transimpedance amplifier; and means for supplying the first reference current value to the second transimpedance amplifier when the data input signal comprises a data one, for deriving the first reference value from said first reference current value, and for supplying the second reference current value to the second transimpedance amplifier when the data input signal comprises a data zero, for deriving the second reference value from said second reference current value.

16. A signal processing element including the control circuit as claimed in any preceding claim.

17. A transceiver for an optical fibre network including the control circuit as claimed in any of claims 1 to 15.

18. An optical drive circuit, comprising: a control circuit as claimed in any of claims 1 to 15; and a laser sub-module, comprising a laser diode and a photodiode optically coupled thereto.